Equipment for measuring gas flow rate

ABSTRACT

The flow detecting elements are provided at the sub-passage for making the part of fluid to be measured (gas) flow. The wall of the sub-passage contains a leak hole (through hole) to drain a liquid having entered and accumulated inside the sub-passage. A protrusion for generating a dynamic pressure on the opening is arranged close to the opening of the leak hole on the external surface of the sub-passage. Alternatively, a protrusion located upstream from the leak hole is formed on the inner wall surface of the sub-passage. The former protrusion generates a dynamic pressure in response to the flow velocity of the gas flowing along the external surface of the sub-passage. The latter protrusion produces a separation flow area for separating the flow from the internal surface of the sub-passage (close to the leak hole), whereby the pressure of the separation floe area is reduced. This configuration provides almost the same pressure differences on the openings of the internal and external surfaces of the sub-passage, thereby reducing the amount of gas leaked from the leak hole. This reduces changes in the distribution flow velocity in the sub-passage between the cases where the leak hole is blocked and where not blocked, and minimizes flow measurement errors.  
     Further, to prevent a liquid film or drop from being formed on the opening by surface tension, structural means is provided close to the opening of the leak hole on the external surface of the sub-passage.

FIELD OF THE INVENTION

[0001] The present invention relates to a gas flow measurement apparatusfor measuring a gas flow rate.

[0002] It particularly relates to a gas flow measurement apparatushaving flow detecting elements in a sub-passage through which part ofthe gas to be measured passes.

[0003] For example, it relates to a thermal resistor type air flowmeasurement apparatus for measuring the intake air flow rate in anautomobile engine.

BACKGROUND OF THE INVENTION

[0004] As a thermal resistor type air flow measurement apparatus (gasflow measurement apparatus) for measuring the intake air flow rate takeninto the automobile engine, it is well-known that the apparatus isequipped with a main passage in which the fluid to be measured flows anda sub-passage in which part of the fluid flows. The flow detectingelements such as a heating resistor and a thermal sensitive resistor areset in the sub-passage.

[0005] Such a sub-passage type flow rate measurement apparatus canstabilize the flow of the gas by utilizing the sub-passage structure.The flow detecting elements can reduce that the flow detecting elementsare affected by the changes of the flow velocity distribution, by thepulsation flow and the back flow. Further, it can reduce thecontamination of the flow detecting elements, and thereby can reduce thedeterioration of quality of the elements. Further, in this apparatus,the flow detecting elements can be mounted easily on the main passageand protected effectively.

[0006] When, however, the internal configuration of the sub-passagechanged due to adherence of foreign substances, the measuring values ofthe apparatus will be increased over those of the gas flow measuringapparatus without sub-passage. To improve the function of thesub-passage, various modifications and improvements are incorporated inthe shape of the passage, and this tends to complicate the shape of thesub-passage. Especially in order to reduce the pulsating flow and tomaintain the measuring accuracy, the sub-passage is designed to have abent form, with the result that foreign substances tend to deposit inthe bent portion. The most probable trouble is that, depending on themounting angle of the flow measuring apparatus, such a liquid as waterwill accumulate in the sub-passage, or water remains in the bent portionof the curved sub-passage, thereby causing a flow measurement error.

[0007] To solve such problems, technical means are proposed to provide adrain hole for preventing water from remaining in the sub-passage, asdisclosed in the Japanese Application Patent Laid-Open Publication No.Hei 07-139414 and Japanese Application Patent Laid-Open Publication No.Hei 09-273950.

[0008] The flow measuring apparatus provided with such a drain hole isintended to prevent from troubles caused by remaining of water in thesub-passage or submersion of the flow detecting elements in water.

[0009] The so-called leak hole (leak path) including the above-mentioneddrain hole is a small hole that does not sacrifice the function of thesub-passage, and prevents from making water remain in the sub-passage.However, the last remaining liquid drop may remain inside the leak holedue to the surface tension.

[0010] Such a residual liquid drop such as waterdrop blocks the leakhole, and this causes greater changes in air flow velocity distributionthan those when the leak hole is not blocked.

[0011] Such a phenomenon causes deterioration of accuracy andperformances including changes in the measurement value and increase ofoutput noise.

[0012] The present invention has been made to solve these problems. Itprevents water and other liquids from adhering and remaining in thesub-passage, and provides a sub-passage that minimizes the flowmeasuring error caused by liquid drops blocking the leak hole.

DISCLOSURE OF INVENTION

[0013] When water and other liquids have remained in the sub-passage, aflow measurement error will be smaller if such liquids are of smallersize. However, if liquid drops have collected to form big particles orthey have accumulated in a particular portion, changes in the flowdivision ratio will be caused by the changes in flow velocitydistribution of the gas flowing in the sub-passage and changes in theresistance of the sub-passage to the flowing gas. This will result in aflow measurement error.

[0014] Such remaining of liquid in the sub-passage can be avoided byproviding a leak hole for leading the liquid out of the sub-passage sothat the liquid can be drained out of the sub-passage, as describedabove. However, this leak hole must be designed to drain easily theliquid; for example, it must have a specified sectional area.

[0015] On the other hand, to make full use of its functions, thesub-passage must be designed so that gas flows inside as intended by thepassage. The leak hole is the passage that does not meet the object ofthe sub-passage. It must have a structure that makes it hard for liquidto flow. This can be achieved, for example, by reducing the sectionalarea of the leak hole.

[0016] To solve these problems, the present invention proposes thefollowing solutions:

[0017] (1) The first of the present inventions provides a structuralmeans, wherein a leak hole is provided to drain the liquid remained inthe sub-passage, and the gas to be measured flowing in the sub-passagehardly flows through the leak hole.

[0018] In the first invention, a leak hole (through hole) between theinternal surface and the external surface of said sub-passage isprovided en route from the inlet of the said sub-passage to the outletthereof; the openings of said through hole is located on the internalsurface and the external surface of said sub-passage, and a structuralmeans for reducing the amount of the leak gas passing through saidthrough hole is provided close to at least one of said openings.

[0019] This configuration prevents water or other liquids from remainingin the sub-passage due to the leak hole. Further, even if the lastliquid drop remaining subsequent to almost complete removing of liquidremains in the leak hole due to surface tension, and thereby the leakhole is blocked, the measurement errors are prevented as follows. Thedistribution of air velocity in the sub-passage maintains anapproximately the same pattern in any case, since the status prior toblocking of the leak hole is so designed as to reduce the amount of flowthrough the leak hole. This configuration minimizes the difference offlow measurement errors between the cases where the leak hole is blockedand where not blocked.

[0020] (2) The above-mentioned leak hole is formed in such a size andshape that the liquid does not remain in excess of the level where theaffect on the flow measurement accuracy cannot be ignored.

[0021] As an embodiment for reducing the amount of gas passing throughthe above-mentioned leak hole in the sub-passage, the present inventionproposes a structural means that generates the dynamic pressure close tothe opening of the leak hole on the external surface of the sub-passage.The dynamic pressure generates according to the velocity of the gas tobe measured, thereby increasing the pressure of that portion.Alternatively, it is also possible to assume that the portion close tothe opening of the leak hole on the internal surface of the sub-passageis formed as a separation flow area for separating the gas flow from theinternal surface of the sub-passage. Thereby, the pressure of that areais reduced, and the difference of the pressures inside and outside theleak hole is also reduced, the amount of the gas flowing through theleak hole is much reduced.

[0022] (3) To put it more specifically, the size of the leak hole isless than one fifth of the sub-passage outlet area, such that the liquidis kept to remain there by the surface tension when there is a smallamount of the remained liquid. For example, when the liquid consists ofwater, the diameter of the leak hole or the width of the shorter side ofthereof is 1 through 5 mm.

[0023] Further, a protrusion for generating the dynamic pressure isprovided close to the opening of the leak hole on the external surfaceof the sub-passage. This protrusion is intended to generate a dynamicpressure close to the opening of the leak hole on the external surfaceof the sub-passage, according to the velocity of the gas flow throughthe main passage. When the shape and size of the protrusion is properlyset, the dynamic pressure is adjusted so that the pressure differencesgenerated at the openings of the leak hole between the internal surfaceand the external surface of the sub-passage will be almost the same.

[0024] (4) To reduce the amount of the gas passing through the leakhole, a protrusion is arranged upstream from the leak hole, on theinternal surface of the sub-passage. This protrusion is used in such away that the portion close to the opening of the leak hole on theinternal surface of the sub-passage is formed as a separation flow areafor separating the gas flow from the internal surface of thesub-passage. Thereby, the pressure of that area is reduced to the levelalmost equal to the pressure close to the leak hole on the externalsurface of the sub-passage.

[0025] The liquid causing measurement errors is drained out of thesub-passage by the leak hole, on the one hand. For the gas, on the otherhand, the pressures on both sides of the leak hole are almost the same,so that there is almost no flow of gas through the leak hole. That is,there is practically no leak hole for gaseous fluids. This prevents thefunction of the sub-passage from being deteriorated. Further, when theleak hole is blocked by water remaining there due to surface tension orthe like, the gas flow is almost the same as that when the hole is notblocked. This configuration maintains required flow measurementaccuracy.

[0026] (5) According to the second invention, a liquid film removingstructure is provided to ensure that water and other liquids do notremain in the form of a liquid drop or liquid film due to surfacetension or the like in the leak hole. This arrangement avoids a flowmeasurement error that may be occurred when the leak hole is blocked.

[0027] For example, the leak hole is designed to have such a size andshape that do not affect the function of the sub-passage.

[0028] Further, the portion close to the opening of the leak hole on theexternal surface of the sub-passage is provided with the structuralmeans (e.g., a protrusion, plate-formed member and rod-formed member).When this arrangement has been made, the surface of the liquid drop orfilm formed in the leak hole hangs down, and when it comes in contactwith a plate- or rod-formed object, the liquid drop is pulled out by theobject, with the contact angle of the liquid drop on the surface of theobject. Thereby the liquid the liquid drop is prevented from remainingin the leak hole.

[0029] Another embodiment proposes an arrangement wherein the dynamicpressure of gas flowing through the sub-passage is applied to theopening of the leak hole on the internal surface of the sub-passage.Thus, the liquid drop or the liquid film tending to remain in the leakhole is broken by the dynamic pressure of gas.

[0030] For example, a partition wall is formed upstream from the leakhole in the sub-passage to create a flow of gas moving toward theopening surface of the leak hole on the internal surface of thesub-passage. The dynamic pressure caused by flow of gas is formed on theopening surface of the leak hole on the internal surface of thesub-passage. Further, the end upstream from the partition wall isarranged to be the portion where higher pressure in the sub-passage isapplied, thereby increasing the pressure on the opening surface of theleak hole on the internal surface wall surface of the sub-passage.

BRIEF DESCRIPTION OF THE DRAWINGS

[0031]FIG. 1 is a cross sectional view and its partially enlarged viewrepresenting a gas flow measuring apparatus as a first embodiment of thepresent invention where the gas flow measuring apparatus is shown withthe cover removed;

[0032]FIG. 2 is an external view of this embodiment in FIG. 1 as viewedfrom the top (from the upstream side);

[0033]FIG. 3 is an external view of this embodiment in FIG. 1 as viewedfrom the left (bottom view);

[0034]FIG. 4 is an explanatory diagram of a gas flow measuring apparatusas a comparative example, showing how water remains in the sub-passagewithout a leak hole;

[0035]FIG. 5 is an explanatory diagram representing how water remains inthe sub-passage with a leak hole;

[0036]FIG. 6 is a transverse cross sectional view and its partiallyenlarged view representing another form of the first embodiment;

[0037]FIG. 7 is a diagram representing the flow velocity vector of thesub-passage without dynamic plate;

[0038]FIG. 8 is a diagram representing the flow velocity vector of thesub-passage provided with dynamic plate;

[0039]FIG. 9 is a diagram representing the pressure distribution of thesub-passage without dynamic plate;

[0040]FIG. 10 is a diagram representing the pressure distribution of thesub-passage provided with dynamic plate;

[0041]FIG. 11 is a vertical sectional view representing the secondembodiment of the present invention;

[0042]FIG. 12 is a partial side view representing the internal structureof the sub-passage of a gas flow measuring apparatus as a thirdembodiment of the present invention;

[0043]FIGS. 13 and 14 are partial side views representing the internalstructure of the sub-passage as another form of the third embodiment;

[0044]FIG. 15 is a diagram representing the flow velocity vector of thesub-passage without deflecting protrusion as a comparative example ofthe third embodiment;

[0045]FIG. 16 is a diagram representing the flow velocity vector of thesub-passage with deflecting protrusion in the third embodiment;

[0046]FIG. 17 is a diagram representing the pressure distribution of thesub-passage without deflecting protrusion;

[0047]FIG. 18 is a diagram representing the pressure distribution of thesub-passage with deflecting protrusion;

[0048]FIG. 19 is a partial side view of a gas flow measuring apparatusas a fourth embodiment of the present invention;

[0049]FIG. 20 is a bottom view of the same;

[0050]FIG. 21 is a partial side view of a fifth embodiment of thepresent invention; and

[0051]FIG. 22 is a vertical sectional view representing a sixthembodiment of the present invention.

BEST FORM OF EMBODIMENT OF THE PRESENT INVENTION

[0052] The first embodiment of the present invention will be describedhereinafter with reference to FIGS. 1 through 3 and 6. Further, FIGS. 4and 5 will be used to describe the state of water remaining in thesub-passage in the prior art example and the present embodiment.Further, the function of this embodiment will be described based on theresult of flow analysis according to FIGS. 7 through 10.

[0053]FIG. 1 is a cross sectional side view and its partially enlargedview representing a thermal resistor type air flow measurement apparatusfor measuring the air flow rate taken into an automobile engine, whereinthe gas flow measurement apparatus is shown with the cover removed.

[0054] The air flow measurement apparatus (gas flow measurementapparatus) comprises of a main passage 8 through which an intake air asa gas to be measured flows, and a sub-passage 4 through which part ofthe intake air flows. The sub-passage 4 is provided in the main passage8.

[0055] In this embodiment, a heating resistor 1, a thermo-sensitiveresistor 2 and an intake air temperature measuring element 3 are fixedto each terminal 11 so that they are located inside the sub-passage 4,and are electrically connected with an electronic circuit 5.

[0056] The electronic circuit 5 controls the heating temperature of theheating resistor 1, based on the intake air temperature detected by thethermo-sensitive resistor 2. For example, the current (heating current)flowing to the heating resistor 1 is controlled so that the temperaturedifference between the heating resistor 1 and thermo-sensitive resistor2 will reach a predetermined temperature difference level. The amount ofheat radiated from the heating resistor 1 is approximately proportionateto the air flow rate, so the electric signal corresponding to the airflow rate can be sent via a connector 9 to the external equipment bydetecting the heating current. In the present embodiment, this method isused to measure the air flow rate. It is also possible to locatethermo-sensitive resistors on upstream and downstream from the heatingresistor, and to measure the air flow rate based on the temperaturedifference between the two resistors. There is no restriction on themethod of measurement.

[0057] The signal of intake temperature measuring element 3 can be usedfor temperature correction of the thermo-sensitive resistor 2 or forother purposes.

[0058] A housing 6 is a plastic component molded with inserting suchmetallic components as the terminal 11. It is an united formation of acase member 6 a forming a frame for incorporation and protection of theelectronic circuit 5, a connector 9 for electric connection with anexternal device, and a flange member 10 for fixing onto a member 7constituting the main passage 8.

[0059] The sub-passage 4 is a plastic molded product, and is connectedto the housing 6 or is formed integrally with the housing 6. Thus, thehousing 6 and sub-passage 4 are integrally structured in parallelarrangement.

[0060] The electronic circuit 5 is installed inside the case member 6 aof the housing 6 and is protected by attaching a cover 12 to the housing6 so as to cover the case member 6 a, as shown in FIG. 2.

[0061] Accordingly, the electric circuit, the detecting elements, thesub-passage and the connector are formed in an integral module.

[0062] The sub-passage 4 is serially formed by;

[0063] an air flow inlet 401 opening on the plane approximatelyperpendicular to the main flow direction 13 of air passing through themain passage;

[0064] a first flow path 402 parallel to the main flow direction 13;

[0065] a first bent portion 403 bending approximately perpendicular tothe main flow direction 13;

[0066] a second bent portion 404 further bending approximatelyperpendicularly;

[0067] a second flow path 405 which is arranged in parallel with thefirst flow path 402, and lets the air flow turned over through thesebent portions pass;

[0068] an air flow outlet 406 opening on the plane approximatelyparallel to the main flow direction 13 at the downstream of the path 405(it's located in an upper side in the main flow direction 13). Thus thesub-passage 4 forms a U-shaped circuitous passage.

[0069] The heating resistor 1, the thermo-sensitive resistor 2 and theintake temperature measuring element 3 are provided in the first flowpath 402.

[0070] Further, in the sub-passage 4, a leak hole (through hole) 407 anda plate-formed projection (hereinafter referred to as “dynamic pressureplate”) 408, which are major points of the present invention, areprovided at the bent portion 404. They are formed integrally with thewall of the sub-passage 4.

[0071] The leak hole 407 opens on the internal surface and the externalsurface of the sub-passage 4, thereby the sub-passage 4 communicateswith the main passage. The cross section of the leak hole 407 can bedesigned in circular, rectangular and many other forms. In the presentembodiment, an oblong slit is used as a leak hole, and its size isdetermined on the order of millimeters. For example, the shorter side isabout 2 mm long. The longer side is set to the width of the sub-passage(See FIG. 3). The size of this leak hole is determined as desired, withreference to sizes of the sub-passage, the main passage and the dynamicpressure plate 408.

[0072] The dynamic pressure plate 408 is located on the plane (base ofthe external surface of the sub-passage) 409 parallel to the main flowdirection on the external surface of the sub-passage 4. It is arrangedclose to the opening of the leak hole 407 on the external surface of thesub-passage. When the upstream and downstream sides in the direction ofthe main flow 13 is defined, with reference to the position of theopening of the leak hole 407 on the external surface of the sub-passage,the dynamic pressure plate 408 is formed so as to protrude from the base409 of the external surface of the sub-passage at the position of thedownstream side.

[0073] This dynamic pressure plate 408 serves as an obstacle to the mainflow 13. When the main flow 13 is received by this obstacle, the dynamicpressure occurs at the opening of the leak hole 407 on the externalsurface of the sub-passage (leak hole outlet).

[0074] In the present embodiment, the dynamic pressure plate 408 isformed so that the surface 408 a facing to the upstream side of the mainpassage is perpendicular to the main flow. The height “h” of the dynamicpressure plate 408 (distance from the opening of the leak hole 407 onthe external surface of the sub-passage to the protruded end of thedynamic pressure plate 408) is set, for example, to about 0.5 through 2times the length W of the short side of the leak hole 407 (diameter ifthe leak hole is circular). Here the optimum value is 2.5 through 3.0mm.

[0075] The dynamic pressure plate 408 is used as a structural means toensure that the leakage of air passing through the leak hole 407 isdecreased by minimizing the difference between the pressure close to thesecond bent portion 404 in the sub-passage 4 (on the opening of the leakhole 407 on the internal surface of the sub-passage) and the pressure onthe opening of the leak hole 407 on the external surface of thesub-passage.

[0076] The main passage 8 is used as the flow path through which the airto be measured passes. In the case of a car engine, for example, itcorresponds to the intake pipe located from an air cleaner to theupstream of the engine cylinder. In the thermal resistor type air flowmeasurement apparatus for automobile, the member 7 constituting thismain passage 8 is located on the midpoint of the intake pipe for thebody of the measurement apparatus use only in some cases. The member 7may be used both as the main passage 8 and an air cleaner or duct orthrottle body etc. in other cases.

[0077] An insertion hole 15 for positioning a measuring module (themodule comprising of the detecting elements such as the heating resistor1 and the thermo-sensitive resistor 2, the sub-passage 4, housing 6 andothers) to inside of the main passage 8 is provided at the wall of themain passage member 7. The measuring module is provided by fixing thehousing 6 at the main passage member 7 with a screw or the like. Thus,the amount of the intake air passing through the main passage can bemeasured.

[0078] In the case of an air flow measurement apparatus without theabove-mentioned leak hole 407, if water from the outside enters tosub-passage 4, water may remains in the sub-passage 4, depending on theinstallation angle.

[0079]FIG. 4 shows how water remains.

[0080]FIG. 4 shows a circuitous sub-passage 4 without a leak hole. Itrepresents the case where the air flow measurement apparatus isinstalled when water is likely to remain in the second bent portion 404(passage bend angle).

[0081] In this case, water entering the sub-passage 4 remains close tothe second bent portion 404. Since the sectional area of the sub-passage4 is reduced close to water area 16, there is an increase in theresistance to air flow in the sub-passage 4, and there is a decrease inthe amount of air flowing through the sub-passage 4 (the air flow rateof the main passage is increased and the air flow rate of thesub-passage is decreased, even if the flow of air to be measured is thesame). Thus, a large negative error occurs in the air flow rate measuredby the air flow measurement apparatus.

[0082] If the amount of the remained water increases by accumulatingaccording to the shape or the installation angle of the sub-passage, thesub-passage 4 will be partially blocked with water. Thereby the air cannot flow into the sub-passage 4, and the air flow cannot be measured.That is, it is also possible that the air does not pass through thesub-passage 4, and the output to indicate the air flow rate becomes zerodue to error.

[0083] Further it is also possible that, if detecting elements 1, 2 and3 are soaked in water, a serious measurement error occurs and thedetecting elements 1, 2 and 3 are damaged by corrosion and electrolyticcorrosion, thereby the measurement is disabled.

[0084] By contrast, in the case of a sub-passage 4 with leak hole 407,water entering the sub-passage 4 flows out through the leak hole 407 asshown in FIG. 5. Thus, the situation where the flow of air in thesub-passage 4 is shut off can be prevented. According to theinstallation angle and shape of the sub-passage 4A, plural leak holes407 may be provided at the position where water may remain.

[0085] When the amount of water entering the sub-passage 4 is muchreduced as shown in FIG. 5, the surface tension in the leak hole allowswater to remain in the state of waterdrop 16 or water film. When theamount of water 16 is increased, it gets stronger than the surfacetension, and water flows out through the leak hole 407. In this case,water also remains in the state of waterdrop or water film when theamount of water becomes very little.

[0086] As described above, in the sub-passage 4 provided with a leakhole 407, water does not remain so much that it hinders the flow of airin the sub-passage 4 even if water enters to the sub-passage 4. However,the leak hole 407 is blocked by water 16. In this case, there is noleakage of air flowing out of the sub-passage (main passage) through theleak hole 407. Accordingly, there is a change in the flow of air in thesub-passage 4 as compared to the case where the leak hole 407 is notblocked by water. Such a situation causes an error in air flowmeasurement.

[0087] In this embodiment, such an air flow measurement error can bereduced as follows.

[0088] The position close to the second bent portion 404 in thesub-passage 4 (close to the opening of the leak hole 407 on the internalsurface of the sub-passage) is an area where dynamic pressure isgenerated. The pressure in the area is comparatively high in thesub-passage 4. On the other hand, the position close to the opening ofthe leak hole 407 on the external surface of the sub-passage belongs tothe low pressure area due to the velocity of the main flow 13 along theexternal wall of the sub-passage if there is no dynamic pressure plate408. However, in case of providing a dynamic pressure plate 408, dynamicpressure is generated. Thereby, since the pressure difference betweenthe opening of the leak hole 407 on the external surface of thesub-passage and the area close to the second bent portion 404 is almostequivalent, the air leakage in the sub-passage 4 is decreasedeffectively even if the leak hole 407 is not blocked by water.

[0089] Accordingly, since there is not much change in the distributionof the air flow between the case where the leak hole 407 is blocked withwater and the case where it is not blocked, air flow measurement errorsare minimized.

[0090] In FIG. 1, the dynamic pressure plate 408 is formed so that thesurface 408 a facing toward the upstream (air inlet side) of the mainpassage 8 is perpendicular to the main flow 13. However, instead of it,as shown in FIG. 6, the surface 408 a facing toward the upstream of themain passage 8 may incline toward the direction of forming an acuteangle to the external surface of the sub-passage (direction of facingslightly toward the side of the leak hole 407). Thus, dynamic pressurecan be generated effectively to the opening of the leak hole 407 on theexternal surface of the sub-passage by forming an inclined surface onthe dynamic pressure plate 408.

[0091] In case of the dynamic pressure plate 408 in FIG. 6, even if theheight “h” of the plate 408 is shorter than that of the dynamic pressureplate 408 in FIG. 1, it can generate the dynamic pressure equivalent tothe plate of FIG. 1. In case of the embodiment in FIG. 6, the optimumvalue of the height “h” of the dynamic plate can be set to 0.5 through2.0 mm when the short side of the leak hole 407 is 2 mm.

[0092] The advantage of the dynamic pressure plate 408 of the presentembodiment will be described with reference to the flow analysis resultgiven in FIGS. 7 through 10.

[0093]FIG. 7 is a diagram representing the flow velocity vector in thecomparative example of the sub-passage 4 (although it has the leak hole407 but it does not have the dynamic pressure plate 408). It shows theresult of the flow analysis when the average flow velocity of the mainpassage (main flow) is 2 m/s. In this case, the flow of velocity of 1m/s or more occurs in the area ranging from the leak hole 407 to themain passage.

[0094]FIG. 8 is a diagram representing the flow velocity vector in thesub-passage 4 provided with the dynamic plate 408 of the presentinvention (the plate 408 whose surface facing the upstream side of themain flow forms an inclined surface), FIG. 8 is drawn by the graphcorresponding to FIG. 6.

[0095] In this case, the pressure difference on the openings of the leakhole 407 on the internal surface and the external surface of thesub-passage, namely both ends of the leak hole 407, can be reduced bythe function of the dynamic pressure plate 408. Thereby, the flowvelocity of air flowing out from the leak hole 407 is reduced below 0.5m/s.

[0096]FIG. 9 is a diagram representing the pressure distribution insideand outside the sub-passage surrounding the leak hole 407 in thecomparative example given in FIG. 7. FIG. 10 is a diagram representingthe pressure distribution inside and outside the sub-passage surroundingthe leak hole 407 in the embodiment given in FIG. 8. FIGS. 9 and 10 showthe result of the flow analysis when the average flow velocity in themain passage is 2 m/s.

[0097] As shown in FIG. 9, when the leak hole 407 is not provided withthe dynamic plate, the pressure on the opening 4 of the leak hole 407 onthe internal surface of the sub-passage is negative. The pressuredifference on the openings of the leak hole 407 on the internal surfaceand the external surface of the sub-passage 4 is 0.5 Pa or more.

[0098] On the other hand, in FIG. 10, the dynamic pressure is generatedon the opening of the leak hole 407 on the external surface of thesub-passage, by the dynamic pressure plate 408. Thereby, the pressure onthe opening is increased. That is, the dynamic pressure is generated inresponse to the air flow close to the side wall 409 of the sub-passageby the plate 408, around the opening of the leak hole 407 on theexternal surface of the sub-passage. The dynamic pressure increases thepressure close to the opening of the leak hole 407 on the externalsurface of the sub-passage. Accordingly, the pressure difference on theopenings of the leak hole 407 on the internal surface and the externalsurface of the sub-passage 4 is reduced below 0.2 Pa.

[0099] In such a configuration, the difference between the maximumpressure close to the opening of the leak hole 407 on the internalsurface of the sub-passage and the minimum pressure close to the openingon the external surface of the sub-passage is kept at a level notexceeding the one fifth of the u²/2 g. The u²/2 g is a value of thedynamic pressure generated by the average flow velocity “u” of the gasto be measured.

[0100] Further, the difference between the detected flow rates where theleak hole is blocked and where not blocked can be made 2% or less.

[0101] The second embodiment will be described with reference to FIG.11:

[0102]FIG. 11 is a sectional view representing a thermal resistor typeair flow measurement apparatus having the sub-passage 4 which is notbent. The sub-passage 4 has a flow contraction section 4′ formed betweenthe inlet opening 401 and the outlet opening 406. The flow detectingelements 1 and 2 are set downstream from the flow contraction section4′.

[0103] In such a sub-passage, water may remain in the flow contractionsection 4′, so the leak hole 407 is provided on the bottom of thesection 4′, and the dynamic pressure plate 408 is formed downstream fromthe leak hole 407.

[0104] In the air flow measurement apparatus of the present embodiment,when water has entered the sub-passage 4, the remained water area 16 islikely to be formed close to the bottom of the flow contraction section4′ in some cases, if there is no leak hole 407. The remained water area16 may hinder air flow. It may change the flow distribution of the flowvelocity at the location of the flow detecting elements 1 and 2, or mayaffect the flow division ratio of the sub-passage 4.

[0105] In the present embodiment, accumulating of water in thesub-passage 4 can be avoided by the leak hole 407. When the leak hole407 is blocked by the waterdrop or the water film, the flow measurementerror may occur. However, according to the same dynamic pressuregeneration principle as that of the aforementioned embodiment, there isnot much change in the distribution of air flow velocity between thecases where the leak hole is blocked and where not blocked. Thisarrangement can reduce air flow measurement errors.

[0106]FIG. 12 is a partial cross section representing a third embodimentof the present invention. The basic configuration of the air flowmeasurement apparatus is the same as that given in FIG. 1. Namely, inthe present embodiment, similarly to the embodiment given in FIG. 1, theheating resistor 1, the thermo-sensitive resistor 2 and the intaketemperature measuring element 3 are fixed to the terminal 11 so thatthey are located inside the sub-passage 4, and are electricallyconnected with the electronic circuit 5. The sub-passage 4, theelectronic circuit 5, the connector 9, the housing 6, the cover 12 andthe main passage 8 are omitted in FIG. 11.

[0107] The major shape of the sub-passage 4 is the same as that ofFIG. 1. The leak hole 407 and the dynamic pressure plate 408 areprovided.

[0108] In the present embodiment, in addition to the above, a protrusion411 having a triangular cross section (angular form) is provided closeto the position upstream from the leak hole 407 in the sub-passage 4.The protrusion 411 has inclinations that faces upstream and downstreamof the sub-passage 4. The protrusion 411 may have one inclination. Theprotrusion 411 deflects the flow of air passing through the sub-passage4. This is called the deflecting protrusion 411.

[0109] The deflecting protrusion 411 is formed on the internal surfaceclose to the second bent portion 404 in the sub-passage 4. In otherwords, here, when the upstream and the downstream in the sub-passage aredefined on the basis of the opening of the leak hole 407 on the internalsurface of the sub-passage, the deflecting protrusion 411 is formedupstream from the opening. The protrusion 411 is close to the opening ofthe leak hole 407 on the internal surface of the sub-passage.

[0110] Accordingly, the direction of the air flowing in the sub-passage4 close to the wall surface of the deflecting protrusion 411 is furtherchanged toward the outlet opening 406 of the sub-passage. Thus, the areaclose to the opening of the leak hole 407 on the internal surface of thesub-passage becomes an area for separating the air flow from theinternal surface. Thereby, the pressure of that area is reduced.

[0111] As a result, in addition to the aforementioned function of thedynamic pressure plate 408, the pressure close to the opening of theleak hole 407 on the internal surface of the sub-passage becomes almostequal to the pressure close to the opening on the external surface ofthe sub-passage. Thereby, the air flow which flows through thesub-passage, when the leak hole 407 is blocked and when not blocked, ismaintained at the almost same state. Accordingly, the measurementerrors, that may be caused by water 16 in the form of the waterdrop andthe water film remaining in the leak hole 407, can be reduced.

[0112]FIG. 13 shows another form of the embodiment given in FIG. 12. Thedifference from FIG. 12 is that the dynamic pressure plate 408 isprovided with an inclined surface similar to the one given in FIG. 6.

[0113]FIG. 14 also shows another form of the embodiment given in FIG.12. The difference from FIG. 12 is that the dynamic pressure plate 408is not provided, and only the deflecting protrusion 411 is used toreduce the pressure close to the opening of the leak hole 407 on theinternal surface of the sub-passage. Thereby this pressure approachesthe level of the pressure close to the opening of the leak hole 407 onthe external surface of the sub-passage.

[0114] The effect of the deflecting protrusion 411 in the presentembodiment will be described with reference to the flow analysis resultgiven in FIGS. 15 through 18.

[0115]FIG. 15 is a diagram representing the flow velocity vector in thesub-passage, wherein the sub-passage 4 is used for the air flowmeasurement apparatus of the embodiment of FIG. 6 (the sub-passage isprovided with the leak hole 407 and the dynamic pressure plate 408, butnot with a deflecting protrusion 411). It is represented in the form ofa cross section parallel to the main flow, and shows the result of theflow analysis when the average flow velocity of the main passage is 25m/s.

[0116]FIG. 16 is a diagram representing the flow velocity vector in thesub-passage, wherein the sub-passage 4 is used for the air flowmeasurement apparatus of the embodiment given in FIG. 13 (provided withthe leak hole 407, the dynamic pressure plate 408 and the deflectingprotrusion 411). It is represented in the form of a cross sectionparallel to the main flow, and shows the result of flow analysis whenthe average flow velocity of the main passage is 25 m/s.

[0117] As shown in FIG. 15, when there is no deflecting protrusion 411,the velocity of the fluid flowing out from leak hole 407 is on the orderof 15 m/s even when the advantage of the dynamic pressure plate 408 isutilized. By contrast, when the deflecting protrusion 411 is provided,the velocity of the fluid flowing out from the leak hole 407 is reducedto about 7 m/s. This ensures a further reduction of the flow measurementerrors even when a leak hole is provided.

[0118]FIG. 17 is a diagram representing the pressure distribution aroundthe leak hole 407 of the sub-passage 4 in the air flow measurementapparatus given in FIG. 15. FIG. 18 is a diagram representing thepressure distribution around the leak hole 407 of the sub-passage 4 inthe air flow measurement apparatus given in FIG. 16. They show theresult of the flow analysis when the average flow velocity of the mainpassage is 25 m/s.

[0119] As shown in FIG. 17, when the deflecting protrusion 411 is notprovided, the pressure difference between the openings of the leak hole407 on the internal surface and external surface of the sub-passage 4 is40 Pa or more. As shown in FIG. 18, when a deflecting protrusion 411 isprovided upstream from the leak hole 407 in the sub-passage 4, thepressure close to the opening of the leak hole 407 on the internalsurface of the sub-passage is reduced. The pressure difference betweenthe openings of the leak hole 407 on the internal surface and theexternal surface of the sub-passage is on the order of 25 Pa.

[0120] The aforementioned embodiments intend to minimize the flowmeasurement error even when water remains in the leak hole 407 due tothe surface tension and others, and even when the leak hole is blocked.

[0121] In the embodiments of FIG. 19 and thereafter, they describe astructure for eliminating liquid films from the leak hole. Theseembodiments actively prevent water from remaining in the leak hole 407due to surface tension and others, so the flow measurement errors willnot occur due to the leak hole being blocked.

[0122]FIG. 19 is a cross section showing the major portion of an airflow measurement apparatus as a fourth embodiment of the presentinvention. The electronic circuit 5, the connector 9, the housing 6,cover 12 and main passage 8 as shown in FIG. 1 are omitted. They havethe same configuration as those of the aforementioned embodiments. FIG.20 shows the bottom view thereof.

[0123] The arrangements of the flow detecting elements such as heatingresistor 1 etc. and overall profile of the sub-passage 4 in the presentembodiment are the same as those of the embodiments described withreference to FIGS. 1 through 18, so they will be omitted in thefollowing description to avoid duplication.

[0124] In the present embodiment, a means for preventing the liquid filmbeing formed on the opening by surface tension is provided close to theopening of the leak hole 407 on the external surface of the sub-passage.

[0125] A plate-formed member 412 (or a rod-formed member) facing thisopening, for example, is used as the liquid film preventing means. It islocated at a position just coming out of the opening of the leak hole407 on the external surface of the sub-passage.

[0126] The plate-formed member 412 is integrally formed with aprotrusion piece 420 additionally provided on the bottom 409 of thesub-passage 4. The protrusion piece 420 is formed integrally with thesub-passage 4 so that it extends at a long side of the body of thesub-passage 4. The plate-formed member 412 is also intersects at rightangles with the protrusion piece 420.

[0127] The plate-formed member 412 (or the rod-formed member) is locatedon the extension approximately of the center line of the opening of theleak hole 407 on the external wall surface of the sub-passage.

[0128] Further, a gap G is provided between the end plane of theplate-formed member 412 and the opening of the leak hole 407 on theexternal surface of the sub-passage. This is because the liquid filmcannot be removed from the leak hole 407, if the end of the plate-formedmember 412 has reached the opening of the leak hole 407 on the externalsurface of the sub-passage or it is inserted into the leak hole beyondthe opening. Since a certain gap G is kept between the opening of theleak hole 407 on the wall surface of the sub-passage and the end of theplate-formed member 412, it is easy to break the liquid film as follows.That is, since the waterdrop comes into contact with the protrusion 412as it hangs down from the outlet side opening of the leak hole 407, thewaterdrop breaks easily.

[0129] When water has accumulated in the sub-passage 4, the leak hole407 drains water away to the outside. In the case of a leak hole havingthe hole size that does not deteriorate the function of the sub-passage4, water in the form of waterdrop and water film remains in the leakhole 407, as shown in FIG. 5. The flow of air through the sub-passage isaffected, as compared to the case where no water remains, andmeasurement errors will occur, as described earlier.

[0130] In the present embodiment, when the waterdrop have grown in theleak hole 407, its surface contacts the plate-formed member 412, and thewaterdrop is pulled out toward the plate-formed member by the contactangle of the waterdrop to the surface of the plate-formed member 412.Therefore, the waterdrop large enough to block the leak hole 407 comesin contact with the plate-formed member 412 and tends to come out alongthe plate-formed member 412. Accordingly, it is possible to preventwater from remaining in the leak hole 407.

[0131]FIG. 21 shows an embodiment that provides a structure forpreventing the water film and the water drop in the leak hole 407,similarly to FIG. 19.

[0132] In the present embodiment, a partition wall 413 is formedintegrally with the sub-passage 4. The partition wall 413 separates theflow of gas around the second bent portion 404 of the internal surfaceof the sub-passage 4, and leads part of the flow to the leak hole 407.

[0133] The separated passage 415 formed by the partition wall 413 andthe internal surface of the sub-passage 404 is located upstream from theleak hole 407. The partition wall 413 ends immediately before the leakhole 407, and the ended portion 414 communicates with the second bentportion 404. The ended portion 414 allows smooth feeding of waterthrough the separated passage 415 formed by the partition wall 413,thereby leading it to the leak hole 407.

[0134] According to the aforementioned configuration, the partition wall413 separates part of air flowing through the sub-passage 4, and leadsthe flow of air straight into the leak hole 407, whereby dynamicpressure is generated on the opening of the leak hole 407 on theinternal surface of the sub-passage. In particular, the upstream end ofthe partition wall 413 is located so as to face the bent portion havinga higher pressure in the sub-passage 4. This configuration can furtherincrease the pressure generated on the opening of the leak hole 407 onthe internal surface of the sub-passage.

[0135] As described above, the leak hole 407, having a small openingarea without affecting the function of the sub-passage 4, allows thewaterdrop and the water film to drain to the outside by the pressuregenerated on the opening of the leak hole 407 on the internal surface.Since the leak hole is open at all times (without being blocked byliquid), it is possible to prevent the flow measurement errors that maybe caused by change of gas flow resulting from the leak hole beingblocked.

[0136]FIG. 22 shows an embodiment wherein a deflecting protrusion 411and the dynamic pressure plate 408 are formed onto the flow measurementapparatus having the same configuration as that of FIG. 11. In such aconfiguration, the direction of air flowing through the sub-passage 4close to the leak hole 407 is changed toward the center of thesub-passage 4, similarly to the case of the embodiment described withreference to FIG. 11. Thus, the portion close to the opening of the leakhole 407 on the internal surface becomes an exfoliation flow area, wherethe pressure is reduced, accordingly the pressures inside and outsidethe leak hole 407 are almost the same with each other. Thisconfiguration allows almost the same flow to be kept between the caseswhere the leak hole 407 is blocked and where not blocked. Thereby themeasurement errors, that may be caused by the waterdrop and the waterfilm remaining in the leak hole 407, are reduced.

[0137] If the area around the leak hole of the aforementionedsub-passage, the protrusion, the plate- or the rod-formed member or thepartition wall for generating the dynamic pressure or the exfoliationarea has a surface coarser than other portions, the following effect canbe expected. According to the configuration, since the contact angle ofliquid on their surface is reduced, water can be easily drained from theleak hole.

[0138] In the aforementioned embodiments, we could get preferableresults when the diameter of the leak hole 407 or the length of theshort side was 0.5 through 2 times the height of the liquid dropproduced by the surface tension of the liquid such as water entering thesub-passage.

INDUSTRIAL FIELD OF APPLICATION

[0139] The present invention can prevent water from remaining in thesub-passage by making water drain through the leak hole, even if watertends to accumulate due to entering into the sub-passage or due tocondensing inside the sub-passage. Further, the present inventionreduces a flow measurement error, independently of whether or not theleak hole is blocked by a liquid film or liquid drop (adhering to theleak hole without remaining in the sub-passage) formed therein. Or theinvention prevents the liquid film such as water film from accumulatingin the leak hole at all times, thereby reducing the flow measurementerror and improving the flow measurement accuracy. Moreover, the presentinvention provides such advantages without affecting the cost, size,weight and others.

What is claimed is:
 1. A gas flow measurement apparatus comprising amain passage for making a gas flow, a sub-passage for making part of thegas flow, and gas flow detecting elements which are provided at saidsub-passage for detecting a gas flow rate; said gas flow measurementapparatus characterized in that a through hole between the internalsurface and the external surface of said sub-passage is provided enroute from the inlet of the said sub-passage to the outlet thereof; theopenings of said through hole is located on the internal surface and theexternal surface of said sub-passage, and a structural means forreducing the amount of the leak gas passing through said through hole isprovided close to at least one of said openings.
 2. A gas flowmeasurement apparatus comprising a main passage for making a gas flow, asub-passage for making part of the gas flow, and gas flow detectingelements which are provided at said sub-passage for detecting a gas flowrate; said gas flow measurement apparatus characterized in that athrough hole between the internal surface and the external surface ofsaid sub-passage is provided en route from the inlet of the saidsub-passage to the outlet thereof, and a dynamic pressure is generatedon the opening of said through hole on the external surface of saidsub-passage.
 3. The gas flow measurement apparatus according to claim 2,wherein when the upstream and the downstream of the main gas flow insaid main passage is defined on the basis of the position of the openingof said through hole on the external surface of said sub-passage, anobstacle to said main gas flow is arranged at the downstream close tothe opening of said through hole on the external surface of saidsub-passage, and said dynamic pressure is generated on the opening ofsaid through hole on the external surface of said sub-passage by saidobstacle.
 4. A gas flow measurement apparatus comprising a main passagefor making a gas flow, a sub-passage for making part of the gas flow,and gas flow detecting elements which are provided at said sub-passagefor detecting a gas flow rate; said gas flow measurement apparatuscharacterized in that: a through hole between the internal surface andthe external surface of said sub-passage is provided en route from theinlet of the said sub-passage to the outlet thereof; when the upstreamand the downstream of the main gas flow in said main passage is definedon the basis of the position of the opening of said through hole on theexternal surface of said sub-passage, a protrusion composing an obstacleto said main gas flow is arranged at the downstream close to the openingof said through hole on the external surface of said sub-passage,wherein said protrusion is formed integrally with the external surfaceof said sub-passage; and a dynamic pressure is generated on the openingof said through hole on the external surface of said sub-passage by saidprotrusion.
 5. The gas flow measurement apparatus according to claim 4,wherein the surface of said protrusion facing the upstream side of saidmain passage is formed perpendicularly to said main gas flow, or A isformed in the state inclined toward the direction of forming an acuteangle to the external surface of said sub-passage.
 6. The gas flowmeasurement apparatus according to claim 4 or 5, wherein the height ofsaid protrusion is about 0.5 through 2 times the diameter of saidthrough hole, or the length of the short side of said through hole.
 7. Agas flow measurement apparatus comprising a main passage for making agas flow, a sub-passage for making part of the gas flow, and gas flowdetecting elements which are provided at said sub-passage for detectinga gas flow rate; said gas flow measurement apparatus characterized inthat a through hole between the internal surface and the externalsurface of said sub-passage is provided en route from the inlet of thesaid sub-passage to the outlet thereof; and structural means is providedin said sub-passage so that the opening of said through hole on theinternal surface of said sub-passage becomes a separation flow area forseparating the gas flow from the said internal surface.
 8. A gas flowmeasurement apparatus comprising a main passage for making a gas flow, asub-passage for making part of the gas flow, and gas flow detectingelements which are provided at said sub-passage for detecting a gas flowrate; said gas flow measurement apparatus characterized in that: athrough hole between the internal surface and the external surface ofsaid sub-passage is provided en route from the inlet of the saidsub-passage to the outlet thereof; when the upstream and the downstreamin said sub-passage are defined on the basis of the opening of said leakhole on the internal surface of said sub-passage, a deflecting structurefor changing the direction of the gas flow in the sub-passage isprovided at the upstream; and the opening of said through hole on theinternal surface of said sub-passage is formed as a separation flow areafor separating the gas flow from the said internal surface by saiddeflecting structure.
 9. The gas flow measurement apparatus according toclaim 8, wherein said deflecting structure is a protrusion formed on theinternal surface of said sub-passage upstream from said through hole.10. The gas flow measurement apparatus according to claim 9, whereinsaid protrusion are inclined on the surface of the upstream side or bothsurfaces the upstream and downstream sides.
 11. The gas flow measurementapparatus according to claim 9 or 10, wherein the height of saidprotrusion is about 0.5 through 2 times the diameter of said throughhole or the length of the short side of said through hole.
 12. A gasflow measurement apparatus comprising a main passage for making a gasflow, a sub-passage for making part of the gas flow, and gas flowdetecting elements which are provided at said sub-passage for detectinga gas flow rate; said gas flow measurement apparatus characterized inthat a through hole between the internal surface and the externalsurface of said sub-passage is provided en route from the inlet of thesaid sub-passage to the outlet thereof, and the difference between themaximum pressure close to the opening of said through hole on theinternal surface of said sub-passage and the minimum pressure close tothe opening on the external surface of said sub-passage is kept at alevel not exceeding the one fifth of the u²/2 g that is a dynamicpressure produced by the average flow velocity “u” of the gas to bemeasured.
 13. A gas flow measurement apparatus comprising a main passagefor making a gas flow, a sub-passage for making part of the gas flow,and gas flow detecting elements which are provided at said sub-passagefor detecting a gas flow rate; said gas flow measurement apparatuscharacterized in that a through hole between the internal surface andthe external surface of said sub-passage is provided en route from theinlet of the said sub-passage to the outlet thereof, and the differencein the detected amount of the gas flow between the cases where the leakhole is blocked and where not blocked does not exceed 2%.
 14. A gas flowmeasurement apparatus comprising a main passage for making a gas flow, asub-passage for making part of the gas flow, and gas flow detectingelement which are provided at said sub-passage for detecting a gas flowrate; said gas flow measurement apparatus characterized in that: athrough hole between the internal surface and the external surface ofsaid sub-passage is provided en route from the inlet of the saidsub-passage to the outlet thereof; if there is water in saidsub-passage, the water can be drained out of said through hole, and astructural means for preventing a liquid film or liquid drop from beingformed due to surface tension is provided close to the opening of saidthrough hole on the external surface of said sub-passage.
 15. A gas flowmeasurement apparatus comprising a main passage for making a gas flow, asub-passage for making part of the gas flow, and gas flow detectingelements which are provided at said sub-passage for detecting a gas flowrate; said gas flow measurement apparatus characterized in that: athrough hole between the internal surface and the external surface ofsaid sub-passage is provided en route from the inlet of the saidsub-passage to the outlet thereof, a plate-formed member or a rod-formedmember is arranged at the position just coming out of the opening ofsaid through hole on the external surface of said sub-passage, and saidplate-formed member or said rod-formed member faces to the opening ofsaid through hole.
 16. The gas flow measurement apparatus according toclaim 15, wherein said plate-formed member or said rod-formed member islocated on the extension approximately of the center line of the openingof said through hole on the external surface of said sub-passage. 17.The gas flow measurement apparatus according to claim 15, wherein a gapis arranged between the end surface of said plate-formed member or arod-formed member and the opening of said through hole on the externalsurface of said sub-passage.
 18. A gas flow measurement apparatuscomprising a main passage for making a gas flow, a sub-passage formaking part of the gas flow, and gas flow detecting elements which areprovided at said sub-passage for detecting a gas flow rate; said gasflow measurement apparatus characterized in that: a through hole betweenthe internal surface and the external surface of said sub-passage isprovided en route from the inlet of the said sub-passage to the outletthereof, a partition wall is formed inside said sub-passage, and saidpartition wall separates the gas flow in said sub-passage and leadingpart of the gas flow to the through hole.
 19. The gas flow measurementapparatus according to claim 18, wherein said partition wall endsimmediately before said through hole, and the ended portion communicateswith said sub-passage.
 20. The gas flow measurement apparatus accordingto claim 18 or 19, wherein said sub-passage contains a section forgenerating dynamic pressure through the gas flowing through saidsub-passage, and said partition wall is formed from the dynamic pressuregenerating section toward said through hole.
 21. The gas flowmeasurement apparatus according to any one of the claims 1 through 20,wherein said through hole is a drain hole for draining such a liquid aswater having entered said sub-passage.
 22. The gas flow measurementapparatus according to any one of the claims 1 through 21, wherein saidsub-passage contains at least one bent passage portion, and said throughhole is arranged close to the bent passage portion.
 23. The gas flowmeasurement apparatus according to claim 22, wherein said sub-passage isformed in a U-like configuration and said through hole is provided inthe corner of the bent passage portion or in its vicinity.
 24. The gasflow measurement apparatus according to any one of the claims 1 through23, wherein said through hole is a slit having the length approximatelythe same as the width of said sub-passage.
 25. The gas flow measurementapparatus according to any one of the claims 1 through 24, wherein thediameter of said through hole or the length of the short side of saidthrough hole is 0.5 through 2 times the height of the liquid dropproduced by the surface tension of the liquid such as water enteringsaid sub-passage.
 26. The gas flow measurement apparatus according toany one of the claims 3 through 6, 8 through 11, and 15 through 20,wherein said sub-passage and said through hole are formed integrallywith said obstacle, said deflecting structure, said plate-formed member,said rod-formed member or said partition wall by plastic molding. 27.The gas flow measurement apparatus according to any one of the claims 1through 26, wherein at least one of the surrounding area of said throughhole of said sub-passage, said obstacle, said deflecting structure, saidplate-formed member, said rod-formed member and said partition wall hasa surface coarser than other portions.
 28. The gas flow measurementapparatus according to claim 27, wherein said coarse surface is formedof numerous fine grooves.
 29. A thermal resistor type air flowmeasurement apparatus for measuring the intake air flow rate in aninternal combustion engine, characterized by the configuration describedin any one of the claims 1 through
 28. 30. An internal combustion enginecontrol system comprising a thermal resistor type air flow measurementapparatus according to claim 29.